trans-Cr(CO)4(PR3)2 derivatives have been determined for R = Et and NMe2. These reactions proceed first by exchange of one carbon monoxide ligand, generating the monosubstituted hydridotetracarbonylchromates K+[HCr(CO)4PR3]-, observed and characterized by NMR spectroscopy for R = Et and Ph. The second step involves substitution of the hydride ligand of K+[HCr(CO)4PR3]- to give the disubstituted derivatives trans-Cr(CO)4(PR3)2